A conventional power conversion device is provided with a voltage conversion unit, which steps up or steps down the input voltage, and constitutes a power source system, which is inclusive of an input power source and an electric storage device. The voltage conversion unit is composed of switching elements, rectifying devices, and reactors, and steps up or steps down the input voltage of direct current, by repeatedly driving the switching elements to turn on and off. Moreover, in the power conversion device, a voltage conversion unit is configured to be in multi parallel constitution, in order to convert the input voltage of direct current into a predetermined output voltage. The voltage conversion unit, which is connected in multi parallel constitution, uses a current sensor and a voltage sensor to control its output, so that overcurrent and overvoltage may not be caused. In addition, the voltage conversion unit performs controlling in step up and step down, so that its output may not exceed standard values of the step up voltage and the step down voltage.
In a multi-phase converting unit, which includes a plurality of voltage conversion units connected in parallel, the voltage conversion units are controlled, so that the number of voltage conversion units, which are made to stop, may increase gradually. So doing, the multi-phase converting unit reveals characteristics that, if a voltage conversion unit is in a normal state, an output of the multi-phase converting unit will have a proper change before and after the switching, and if a voltage conversion unit is in an abnormal state, an output of the multi-phase converting unit will exhibit an abnormal change before and after the switching. There are shown a failure detection device and a power source device, which use those characteristics and can specify one or more voltage conversion unit(s) which is or are in an abnormal state. (Patent Document 1)